Electrical machines, such as motors and generators having a rotor with permanent magnets are known. They are generally deemed to be reliable and require less maintenance than other generator typologies.
Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a plurality of blades. The rotor with the blades is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft drives the generator rotor either directly (“directly driven”) or through the use of a gearbox. Particularly for offshore wind turbines, direct drive systems employing permanent magnets are usually chosen. However, this type of generator is not limited to offshore applications, and not even to the field of wind turbines only.
Use of a permanent magnet synchronous generator in the power train of a wind turbine has some advantages compared to other types of generators commonly used on wind turbines. These advantages include, inter alia, high efficiency, reduced losses on the rotor of the generator, lack of any slip-rings and their attendant problems, and better low voltage ride through performance.
However, when a short circuit event, such as an arc, occurs in the generator or converter during normal operation, a permanent magnet synchronous generator will continue to generate stator voltage as long as the generator is turning since the permanent magnets in the rotor will continue to produce a magnetic field. Therefore, short circuit currents arise under such conditions and they must be properly handled to avoid damage to the generator (and the wind turbine).
Different types of short circuit events can occur. Short circuit events can be dangerous. For example, such events can lead to fires.
Short circuit events may be handled in different ways in order to reduce impact on the wind turbine. In a three-phase generator and power converter configuration, a power converter failure will in most cases appear as a three-phase short circuit on the generator. Said three-phase short circuit on the generator will cause a transient torque oscillation followed by an almost complete unloading of the drive train. The unloading produces undesirable mechanical load levels on a wind turbine. One way to reduce the impact from such a short circuit event is to disconnect the converter from the generator and subsequently apply a passive dump load, which will serve to reintroduce a certain load torque (i.e. a resistant torque) on the drive train.
Another type of short circuit event is an unbalanced short circuit. When, for example, a two-phase short circuit (which is a type of unbalanced short circuit) occurs in the generator, a transient torque oscillation appears followed by a stationary torque oscillation with a frequency determined by the speed of the generator. One countermeasure that can be taken in response to an unbalanced short circuit in the generator is to shut down the wind turbine and bring it to a standstill.
However, as the rotor slows down and is brought to a standstill, the torque oscillation that depends on the speed of rotation would go through a very large frequency range resulting in a high risk of exciting resonances in the drive train and as the case may be the turbine foundation.
Various wind turbines are known that incorporate some form of countermeasures to overcome problems related to an unbalanced short-circuit. One known possible countermeasure consists of installing specific switching devices between phases of the generator in order to force a three phase fault when a two phase fault is detected reducing the short-circuit torque.
WO 2014/079453A2 discloses a method for reducing an impact of an unbalanced short circuit event that occurs in a polyphase permanent magnet generator of a wind turbine. According to the method, an unbalanced short circuit event is detected in the generator of the wind turbine, and, in response to detecting the unbalanced short circuit event, at least one phase of the generator is shorted at a switch-point between the generator and a converter of the wind turbine to create a balanced short circuit in the generator. By doing so, the torque response of the generator is altered to avoid high amplitude torque oscillations that would other occur as a result of the unbalanced short circuit event.
Another known possible strategy to avoid short-circuit related issues consists of installing protecting relays or switches to the star common points of the three-phase system in order to open the neutral point of the faulty system whenever a short-circuit is detected.
However, none of the above mentioned systems is able to provide high reliability on the detection of the fault. It might occur that a detection failure is produced and then the system switches over to the fault mode during normal operation. Furthermore, only the system forcing a three phase short-circuit is able to reduce the fault torque. However, since it is based on switching devices, a delay in the reaction time takes place.
In examples of the present disclosure, some of the aforementioned problems may be at least partially resolved.